Imagine a whip-smart economist with a sprawling imagination. Now imagine he's 9 years old and wants to know everything. That is the basic profile of Steven Levitt. A University of Chicago economist, Levitt, 37, is in fact an adult. But he has built his name by asking questions packed with curiosity and devoid of judgment: If drug dealers make so much money, why do they still live with their moms? Did crime in the 1990s go down because the number of abortions in the 1970s went up, or is that just a coincidence? Does parenting actually matter when it comes to...